1. Field of the Invention
The present invention relates to a switching power supply unit as a DC/DC converter, and particularly to a switching power supply unit with low power consumption.
2. Description of the Prior Art
In recent years, demands for electric power saving of electronic devices have been increased. To save power in electronic devices, it is important to improve power conversion efficiency of a power supply unit supplying power to the devices in addition to a reduction in the power consumption of the devices. A switching regulator is commonly used for the power supply unit. The switching regulator is configured to convert electric power supplied to an input terminal to a predetermined voltage for outputs.
There are two types of the switching power supply unit as a diode rectification type using a diode and a synchronous rectification type using a rectifying switching element instead of a diode. The switching power supply unit comprises an output switching element, a rectifying diode or a rectifying switching element, an inductor, and a capacitor. Further, there are another four types of switching power supply unit as a buck type which steps down an output voltage from an input voltage, a boost type which steps up an output voltage from an input voltage, a buck-boost type which can step up/down an output voltage from an input voltage, a reverse type which can generate an output voltage in reverse polarity to that of an input voltage.
FIG. 8 shows an example of characteristic curves of a load current to power conversion efficiency of a prior art switching power supply unit having general power conversion efficiency. The characteristic curves are of transistors in three sizes (large, medium, small) used in each switching element. It is apparent from the drawing that peak values or magnitude of the power conversion efficiency differ depending on the size of a transistor used for the switching element in the switching power supply unit.
Various techniques for improving the power conversion efficiency of the switching power supply unit have been proposed. One of such techniques is to optimize charge/discharge currents along with a parasite capacitance of each switching element by changing a transistor size of the output switching element and that of the rectifying switching element in accordance with a load level. For example, Japanese Patent Application Publication No. 2007-20316 (Reference 1) and No. 2007-124850 (Reference 2) disclose a technique to improve the power conversion efficiency by detecting a magnitude of a load current (output current) with a load current detecting circuit and changing transistor sizes of the output switching element and the rectifying switching element depending on a detected value of the load current. In general, the larger the transistor size on an IC chip, the larger the power (allowable current amount) of the transistor, the smaller the on-resistance and the larger the parasite capacitance.
FIG. 7 is a circuit diagram of an example of a prior art switching power supply unit using the above technique (disclosed in Reference 2, for example). A buck-type switching power supply unit 110 in FIG. 7 comprises a large power converter 112, a small power converter 114, an input current detection circuit 118, and an output voltage detection circuit 120. The input current detection circuit 118 switches a converter in use from the large power converter 112 to the small power converter 114, upon detecting that an input current Iin is smaller than a predetermined current value. The output voltage detection circuit 120 switches a converter in use from the small power converter 114 to the large power converter 112, upon detecting that an output voltage VOUT is smaller than a predetermined voltage. Thus, the switching power supply unit switches the large and small power converters 112, 114 in accordance with electric power necessary for a load ROUT.
Further, Reference 2 discloses switching power supply units of a boost type, a synchronous rectification type, and a diode rectification type, for example.
The switching power supply unit 110 in FIG. 7 comprises a resistance 181 for current-voltage conversion connected in series between an input terminal IN and output switching elements QL1, QL2 in order for the input current detection circuit 118 to detect magnitude of the input current Iin. The switching power supply unit 110 is configured to improve the power conversion efficiency by switching the large power converter 112 to the small power converter 114 when required power for a load is low. However, there is a problem that a power loss due to the resistance 181 is non-negligibly large with the power conversion efficiency during a low load state taken into account. To reduce a power loss in the resistance 181, a resistance value thereof has to be reduced. However, with a low resistance value of the resistance 181, a larger current is flowed therein, which makes it necessary to increase a layout size of the resistance 181 on an IC chip. This accordingly causes a problem with increasing the entire circuit footprint on the IC chip on which the switching power supply unit is integrated. Moreover, the input current detection circuit 118 and the output voltage detection circuit 120 incorporated in the switching power supply unit 110 also cause an increase in the entire circuit footprint on the IC chip.
Furthermore, another problem with the switching power supply unit 110 is that since the transistor sizes of the output switching element and the rectifying switching element are automatically selected by a detected load current, a user cannot select an optimal transistor size according to a load in use.